Not sure if this is a bug but let's see. I tried to use s-nail as mail client in Options-Internet-Email and NOK. https://wiki.documentfoundation.org/Faq/General/033, which is wrong for mail merge, says that supported clients are iceape, mozilla, netscape, seamonkey, icedove, thunderbird, kmail, mutt, evolution, gnome, xdg, groupwise, dtmail, sylpheed, claws. Not sure if that's correct, but s-nail is a classic command mail client and it should be supported (like another one: mail).
There's more here, may be resolved here or in another bug. I searched Help for Email and got 2 links. https://help.libreoffice.org/7.4/en-US/text/shared/01/01160200.html https://help.libreoffice.org/7.4/en-US/text/shared/guide/email.html Both say "Opens a new window in your default email program.." and "LibreOfficeDev opens your default email program" but don't say which email programs are supported. Someone looking for that info should find it, and not to go elsewhere to Wiki which explains a difference for Windows (any MAPI compliant email client specified in the Windows Control Panel) and Linux (list of clients). As for list of Linux clients, I didn't test, but some are known (GUI: thunderbird, evolution, kmail, groupwise, sylpheed/claws, COMMAND: mutt), some are of less prominence, but at least some are not clear (gnome, xdg). What is "gnome", Geary or else? "xdg" should be xdg-email. What is "netscape" now :) What exactly is "mozilla", discontinued Mozilla Application Suite? iceape (seamonkey), icedove (thunderbird) were Debian rebranded clients but are long unmaintained. Only SeaMonkey is actual. These may remain in Wiki, but in Help should be currently used clients. Missing is BlueMail.
I guess there is no reason why not to confirm. It just need to be added to senddoc as a case s).